It is known that many polymers can be produced as powders in fluid bed reactors where the fluidization is provided by a circulating mixture of gases including one or more monomers. For example, vapor phase polymerization is a common process, widely used for the production of polyolefins, such as ethylene, and polyolefin copolymers. One particularly arrangement of a fluid bed polyolefin process is disclosed in U.S. Pat. No. 4,882,400. Other examples of fluid bed polyolefin technology are described in, for example, U.S. Pat. Nos. 3,709,853; 4,003,712; 4,011,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749; 5,541,270; 7,122,607, and 7,300,987. These patents disclose gas phase polymerization processes wherein the polymerization medium is either mechanically agitated or fluidized by the continuous flow of gaseous monomer and diluent.
One example of a prior art vapor phase polymerization method is illustrated in FIG. 1. A catalyst is fed through supply line 2 into a fluidized bed reactor 4 and, simultaneously, a gaseous olefin is caused to pass through supply line 6 and blown into the bottom of the fluidized bed reactor 4 through a gas distributor plate 8. The gas distributor plate 8 may include, for example, a perforate plate having a plurality of through holes, and is arranged in the vicinity of the bottom of the fluidized bed reactor 4. In this way, a fluidized bed 10 is formed and held in the fluidized state in the fluidized bed reactor 4. Polymerization of the monomer is carried out in the fluidized bed 10, and polymer particles produced by the polymerization reaction are continuously discharged through line 12. Unreacted gaseous olefin having passed through fluidized bed 10 has its flow rate reduced in a velocity reduction zone 14 provided in an upper part of the fluidized bed reactor 4, where the vapor velocity is reduced so as to avoid or reduce entrainment of polymer particles from fluidized bed 10. The unreacted monomer is discharged outside the fluidized bed reactor 4 through gas outlet 16 disposed at a top of velocity reduction zone 14. The unreacted gaseous olefin is then recycled via line 18 to the bottom of fluidized bed reactor 4 via compressor 20. Monomer added via line 6 accounts for monomer reacted to form the polymer removed via flow line 12, thus maintaining a constant supply of monomer to reactor 4. The heat of polymerization generated in fluid bed 10 may be removed from the system by cooling the recycle gas in heat exchanger 22.
The active, growing powder in a fluidized bed polyolefin reactor, such as that described in FIG. 1 and the aforementioned patents, contains a wide range of particle sizes. Thus, the powder is referred to as having a broad particle size distribution. Some of the reasons for the broad size distribution are the size range of the initial catalyst particles (or prepolymer particles) charged to the reactor, the difference in catalytic activity of each catalyst particle, the difference in residence time for each growing polymer particle, the agglomeration of polymer particles, and the spalling of polymer particles.
The size distribution of particles can be characterized by various physical measurements relating to the particle mass, physical dimensions, or specific surface area. Two commonly used methods of measurement, owing to ease and reproducibility, are mechanical sieve analysis and the light scattering behavior of a cloud of particles. The very small polymer particles are called fines. As used in the art, the term “fines” refers to some defined fraction of the polymer powder particles that are smaller than the average of the entire population of powder particles present in the fluid bed. Particularly small polymer particles, for example, smaller than 125 microns, are considered fines.
In the fluid bed processes for the production of polyethylene and ethylene copolymers, high levels of polymer powder fines in the reactor pose significant and well known operating difficulties. Within the reactor, a higher level of fines often leads to increased agglomeration of the polymer powder. Outside the reactor, the fines may deposit in the recycle system and grow, fouling the piping, heat exchangers, compressors, and the reactor inlet gas distribution grid.
Inside the reactor, fines are a leading contributor to the formation of powder agglomerates. For various reasons, fines tend to segregate into certain poorly circulated and poorly cooled regions of the reactor. Exacerbating the segregation problem is the tendency of fines to have higher than average catalytic activity, causing the fines to be hotter than the average particle. This higher catalytic activity in fines is due to higher concentrations of active catalyst components in the fines and due to short diffusion paths for monomers and co-catalyst molecules.
One undesirable place at which fines accumulate is along the reactor vessel wall in the zone occupied by the main fluid bed. This accumulation is believed to occur because fines are more greatly affected by static forces due to their larger ratio of surface area (static charge) to mass (inertia). Thus, fines can cling by static electric forces to the electrically grounded metal wall of the reactor. Polymerization in the stagnant layer of reactor wall fines releases heat which can lead to melting and fusing of polymer into sheets along the vessel wall. These sheets of fused polymer may grow quite large before coming loose and falling into the fluid bed. Once fallen into the main fluid bed, such sheets can obstruct powder fluidization, circulation, and withdrawal. In some cases, the sheets may be so large as to significantly disrupt the normal fluidization and circulation of gas and solids of the entire fluid bed, leading to extensive fusing of the main bed. When powder withdrawal slows or the bed fuses, the reactor production must be stopped and the reactor vessel opened for cleaning. This is a very costly production outage.
Another undesirable place where fines accumulate is in the velocity reduction section of the reaction vessel. The velocity reduction section (disengaging zone) of the reactor is a region of expanded cross-sectional area that is above the zone in which bed level normally resides. The purpose of the expanded area is to reduce the velocity of the fluidizing gas in order to minimize the entrainment of fine particles in the gas leaving the reactor. In the disengaging section, fines tend to concentrate in the regions of lower gas velocity nearer the downward sloping vessel wall. In fact, it is intended that most of the fines fall onto the sloped wall of the disengaging section and slide downwards and back into the main fluidized bed. However, when the concentration of fines increases in the disengaging section, the polymerization heat load on the sloped wall becomes larger. This can be observed by increasing temperatures seen from indicators placed in the sloped wall in the disengaging section. The concentration of fines and resulting heat can become great enough to lead to melting and fusing of the powder into sheets along the sloped wall. These sheets will tend to grow until their own weight and hydrodynamic forces cause them to fall into the main fluid bed, there, as discussed above, obstructing powder withdrawals and possibly causing more extensive bed fusing. As with the above, when either of these occurs, the reactor production must be stopped and the reactor vessel opened for cleaning.
In addition to problems caused inside the reactor, problems are also caused by fines outside the fluid bed reactor vessel. Some fines will leave the reactor vessel in the overhead piping that carries the recycle gas away for cooling and compression. The exiting fines may attach to surfaces of piping, heat exchangers, and other process equipment in the recycle loop. Recycled fines may also settle in regions of lower gas velocity, such as the bottom of the reactor underneath the distribution grid for the fluidizing gas.
Because fines exiting the reactor retain their catalytic activity, they continue to react outside the reactor. Thus, fines depositing in the recycle system equipment grow and fuse to create skins, sheets, and lumps of polymer. These skins, sheets, and lumps reduce heat transfer efficiency and modify mass flow in the recycle gas piping and equipment. Also, some fines will return to the reactor via the recycle system. Because the temperature and gas composition are very different at some locations in the recycle system, the polymer produced outside the reactor may have very undesirable properties. Although a minute fraction of the total polymer production, the fines returning to the reactor can nonetheless seriously impact the suitability of the overall product. The presence of non-homogeneous polymer fines in the final product can significantly affect the quality of the product and resulting articles produced therefrom, such as the formation of gels in polyethylene films.
Several methods for addressing the operating problems associated with fines discussed above are presented in U.S. Pat. Nos. 4,956,427, 4,882,400, 4,803,251, 4,532,311, 5,126,414, 4,933,149, 5,352,749, 5,428,118, 5,461,123, 6,905,654, European Patent Application Publication EP 453,116 A1 and U.S. Patent Application Publication No. 2008/0027185.
In light of the above, there exists a need in the art for gas-phase reactors that may be operated in a manner to avoid the formation of at least one of polymer sheets, skins, lumps, and agglomerates.